Put the butter with garlic, rosemary, sea salt, black pepper in a bowl, and mix with a rubber spatula until well combined.

Thinly slice baguette into 3mm-thick slices with a serrated knife. Spread some flavored butter on sliced baguette, and place in a single layer. Bake in preheated oven for 9 to 10 minutes or until light brown and crisp. Remove from the oven, and let it cool completely before serving.